Process for producing a plate heat exchanger and plate heat exchanger

ABSTRACT

A plate heat exchanger has two metal plates brought into abutment, with a solder material between the plates. The plates are heated up to a first temperature. The plates are placed into a mold, the mold surfaces of which have cavities for envisaged channel structures. Channel structures are formed by local internal pressure forming of at least one plate under pressurization by the tool. The plates are heated up to a second temperature. The plates are solder bonded at the abutted surfaces. A plate heat exchanger has two metal plates, wherein channel structures have been formed in at least one plate and the plates are bonded to one another by soldering away from the channel structures. Eutectic microstructures having a longest extent of less than 50 micrometers are formed in the solder layer.

RELATED APPLICATIONS

The present application claims priority of German Application Number 102019 120 862.9 filed Aug. 1, 2019, the disclosure of which is herebyincorporated by reference herein in its entirety.

The invention relates to a process for producing a plate heat exchangerand to a plate heat exchanger.

Plate heat exchangers are used, for example, to control the temperatureof storage units for electrical energy in motor vehicles. A known methodis to use a large-area plate heat exchanger to control the temperatureof all storage units arranged a vessel. Likewise known is the control ofthe temperature of individual modules of storage units. In particular,the storage units are to be cooled in operation or in the course ofcharging.

Plate heat exchangers are additionally usable in many other technicalfields and therefore have a wide range of dimensions.

DE 10 2010 051 106 A1 discloses a plate heat exchanger consisting of twoplate elements made of aluminum. Convexities are shaped into the plateelements, and these collectively form at least one cooling channel. Theplate elements are then bonded to one another by welding. Thisproduction process necessitates a separate forming mold. Moreover,welding is an inconvenient and costly process, and the length of theweld seam additionally rises with rising dimensions. The heat input isalso disadvantageous to the strength of the material.

DE 10 2014 219 812 A1 discloses a process in which a separating agent isdisposed between two metal sheets in order to define channel structures.The metal sheets are inextricably bonded to one another in a rollbonding process. Subsequently, the regions treated with separating agentare blown up in order to form the channel structures. In roll bonding,however, dimensional accuracy falls with increasing plate size.Moreover, the selection of possible material pairs for the metal sheetsis limited.

Finally, it is also known that the two plates can be soldered to oneanother in the production of a plate heat exchanger. DE 2012 009 148 B4discloses a component made of two layers of metal, at least one of whichis profiled, so as to form a cavity between the two layers in order toconvey a medium therein. The two layers may also be bonded to oneanother by soldering. It is customary to perform the soldering in anoven through which the components are conveyed. As well as the oven,there are also a forming mold and special fixing frames for thecomponents in the oven, which entails high capital costs. Moreover, thesoldering operation takes 1 to 2 hours, which is associated with longcycle times. This long-lasting thermal stress additionally has thedisadvantage that magnesium precipitates that weaken the solder bondoccur in the case of relatively high-strength aluminum alloys. Thismeans that these materials are unusable for a plate heat exchanger.

It is therefore an object of the present invention to provide a processthat enables the inexpensive, dimensionally accurate and flexible (interms of the materials) production of plate heat exchangers having longlifetime. This object is achieved by a process having the features ofclaim 1. It is a further object of the invention to provide a plate heatexchanger which is inexpensive and flexibly producible in terms of thematerials and has a long lifetime. This object is achieved by a plateheat exchanger having the features of claim 20. Particularconfigurations of the invention are the subject of the respectivedependent claims.

The invention relates to processes for producing a plate heat exchangerhaving the following steps:

-   -   providing two plates of a metallic material, wherein the plates        are brought into abutment with a solder material disposed        between the plates,    -   heating the plates to a first temperature,    -   inserting the plates into a mold having mold surfaces that have        cavities for envisaged channel structures,    -   forming channel structures by local internal pressure forming of        at least one plate under pressurization by the mold,    -   heating the plates to a second temperature,    -   solder bonding of the plates in the abutted areas.

Herein, when the two plates are brought into abutment (or one plateabuts the other plate) with a solder material disposed between theplates, the two plates are in direct contact in at least one regionwhere the solder material is not provided and/or, in at least one regionwhere the solder material is provided, the two plates would have been indirect contact but for the solder material. In some embodiments, theabutment of the two plates with the solder material disposedtherebetween allows for at least a point or a region of direct contactbetween opposing surfaces of the two plates. The plates are notnecessarily being in direct contact across the entirety of theiropposing surfaces, because the solder material in between may preventthe opposing surfaces of the plates from directly touching each other inone or more regions.

The plates together with the solder material disposed between the platesare first heated to a first temperature. This temperature is chosen suchthat the material of at least one of the plates is readily formable, butthe solder material has not yet melted. The first temperature ispreferably 200° C. to 550° C., more preferably 300° C. to 350° C.

In internal pressure forming, a medium is guided between the platesunder high pressure. In the region of the cavities, the material of theplates can deform and fit to the inner surfaces of the cavities, whichcreates hollow channel structures, while the plates remain fully abuttedin the regions outside the cavities. At the same time, the plates areheld in position relative to one another. The process leads in anadvantageous manner, through the reproducible formation of precisechannel structures in a mold, to a highly dimensionally accurate plateheat exchanger.

The internal pressure forming is preferably effected by means of aninert gas, for example nitrogen. The gas may have been preheated.

In the forming of the channel structures, it is also possible tointroduce stiffness-increasing beads into the plate heat exchanger,which are not in fluid connection with the channel structures.

The plates are subsequently heated to a second temperature in which thesolder material melts and the solder bond forms in the abutted areas.This creates a stable bond between the plates. The second temperature isadvantageously 550° C. to 650° C., preferably 600° C. to 615° C.

The plates are preferably heated uniformly to the second temperature intheir regions provided with solder material, where the temperature rangeΔT over the plate is less than 50 K, preferably less than 10 K.

In an advantageous configuration of the invention, the mold is heatableand the heating to the first temperature is effected in the mold. Theplates are heated by means of thermal conduction, which leads toparticularly rapid heating of the plates. Moreover, it is possible tocommence the internal pressure forming directly on attainment of thefirst temperature. It is thus possible to achieve shortening of thecycle times and to dispense with additional devices for heating and fortransport of the plates.

It is also advantageous when the solder bonding of the plates islikewise effected in the mold under pressurization by the mold. Thismeans that the plates, after the internal pressure forming, preferablyremain in the mold and are heated further to the second temperature.This too is effected by the direct contact with the mold within a veryshort time. It is also advantageous here that the plates are positionedso as to be immovable relative to one another, such that no more warpageor slip relative to one another is possible. This likewise leads to highdimensional accuracy in the plate heat exchangers.

The total process duration is a few minutes, which is considerablyshorter than the customary dwell time in a soldering oven of up toseveral hours.

The mold is preferably preheated. More particularly, it is preheated toa temperature of 550° C. to 700° C., preferably to a temperature of 600°C. to 650° C., more preferably to a temperature of 615° C. to 625° C.Immediately after the plates have been inserted into the preheated mold,the plates and the solder material are heated, the speed of heatingbeing proportionate to the temperature of the mold. This too contributesto a reduction in cycle times.

In an advantageous configuration of the invention, sealing beads areprovided adjacent to the cavities of the mold surfaces and/or around themold circumference, in order to seal the resultant channel structures inthe internal pressure forming. This means that the plates are locallypressed against one another at high pressure, such that the internalpressure forming in the region of the cavities is effected precisely andwith high dimensional accuracy.

Advantageously, one plate is provided as base plate, the other plate asforming plate, and the channel structures are generated in the formingplate. This means that just one of the two plates is subjected tointernal pressure forming and the mold has the necessary cavities onlyin one part of the mold. This reduces the cost and inconvenienceassociated with the provision of the mold. Moreover, it is possible toassign an additional function to the base plate, for example as baseprotection sheet.

The base plate preferably has a thickness of 0.5 to 5 millimeters,preferably 1 to 2 millimeters.

The forming plate preferably has a thickness of 0.2 to 2 millimeters,preferably 0.8 to 1 millimeter.

The thickness of the plates also depends on the material used, forexample the specific alloy and/or pretreatment, and its properties.

In a further advantageous version of the invention, the plates consistof an aluminum alloy, preferably of a high-strength aluminum alloy. Theindividual plates here may consist of different alloys. These may bealloys that are typically used in soldering processes, for examplealloys of the 3000 series, for instance 3003 or 3005. However, thealloys may also be of the 5000 series. Alloys of the 6000 and 7000series are generally considered to be difficult to solder, sincemagnesium precipitates form on the surface during the generally longperiod of heating to the soldering temperature. However, the process ofthe invention enables very rapid heating of the plates in the region ofseconds or a few minutes, by contrast to up to several hours inconventional oven soldering processes. This means that the magnesiumprecipitates can also be largely avoided. It is thus possible, by virtueof the process of the invention, to also consider aluminum alloys foruse in a plate heat exchanger that typically cannot be used.Ultrahigh-strength aluminum alloys in turn enable additional functionsthat can be assumed by the plate heat exchanger, for example asstructure component.

Preferably, one of the two plates has been provided with a plated solderlayer. This facilitates the handling of the plates and avoids anadditional operating step in order to apply the solder.

Advantageously, the plates are arranged such that the solder-platedplate is disposed at the top in the mold. During the solderingoperation, diffusion processes toward the lower plate can proceedassisted by gravity and ensure a more stable bond.

In addition, a connection opening is generated in at least one of theplates. This can be effected, for example, by punching, cutting oranother method. The connection opening serves to accommodate aconnecting element through which the cooling medium is later introducedor discharged.

In addition, a connecting element is disposed on or in this connectionopening. Preferably, the connecting element is disposed on or in theconnection opening. More preferably, the connecting element is disposedin the connection opening with a solder ring and/or a solder paste. Inthis way, the connection between connecting element and accompanyingplate can be effected simultaneously with the solder bonding of theplates.

In addition, in a particular configuration of the invention, the mediumfor the internal pressure forming is introduced through the connectionopening. Thus, no additional connections are needed for the introductionof the medium for the internal pressure forming. The connections presentin any case for the introduction or discharge of a cooling medium areutilized.

Advantageously, in the process of the invention, a mold is used, themold surfaces of which have been provided with a coating in order toprevent adhesion of the plates. This may be, for example, a coating ofceramic or other suitable materials.

Together therewith or alternatively, it may be the case that aseparating agent is disposed between the plates and the mold surfaces ofthe mold in order to prevent adhesion of the plates. The separatingagent may, for example, be a suitable fluid which is applied to theplates or the mold surfaces of the mold. An alternative is a film sheetwhich is introduced between plate and mold. An alternative is a wearsheet fixed in the mold, including in a detachable manner.

It is preferably the case that, in the internal pressure forming, afirst pressure is applied by the mold and, in the solder bonding of theplates, a second pressure is applied by the mold, the first pressurebeing higher than the second pressure. In the forming operation, theplates are subjected to higher pressure in order to fix the platesrelative to one another and to generate an opposing pressure to themedium introduced for internal pressure forming, and in order to sealthe channel structures in order to achieve high dimensional accuracy.Thereafter, the pressure applied is reduced, which is associated withthe rising temperature, since high temperatures promote the adhesion.The reduced pressure reduces normal stresses in the plates, which are acause of the adhesion of plate material to the mold.

Such a variation in pressure can be enabled and assisted byspring-mounted molds. In the case of use of spring-mounted molds, it isalso possible to influence the heating operation by altering theconduction of heat between the mold and the plates through a variationin the contact pressure.

After the solder bonding of the plates, they are cooled down again. Thecooling can be effected by contacting with a cooling medium in or afterremoval from the mold. Alternatively, the plate heat exchanger can alsobe transferred into a cooling mold or a cooling frame. Also possible isa combination of these or further process steps.

More particularly, the medium for the internal pressure forming may beused for cooling or for assistance of the cooling process. If the mediumis an inert gas, for example nitrogen, there is the additional advantagethat the formation of an oxide layer is prevented. Moreover, the coolingis effected more evenly and reduces the formation of the plates that canbe caused by internal stresses.

Preferably, the cooling period between the solder bonding and the soldersolidification is shorter than 60 seconds, preferably shorter than 20seconds, more preferably shorter than 10 seconds. The effect of thisshort cooling time is that eutectic microstructures having a longestdimension of less than 50 micrometers form in the solder layer. Inconventional processes, these microstructures form much coarser grainsand form elongate acicular structures having a length of 200 micrometersor more. These form because, in conventional soldering methods, forexample oven soldering, the cooling can take tens of minutes and thestructures develop in this time. However, the effect of the fine-grainmicrostructure is that the solder bond is much more stable andfatigue-resistant. The fine-grain microstructure forms especially in thetransition region between the abutted plates and the channel structures,and prevents premature cracks and the plates from splitting apart inthis region which is highly stressed in operation. This distinctlyincreases the lifetime of plate heat exchangers of the invention.

Further preferably, the cooling is followed within less than 72 hours bya heat treatment at a temperature of 140° C. to 250° C. over a period of20 minutes to 24 hours, preferably of 20 minutes to 8 hours, morepreferably of 20 minutes to 2 hours. In an advantageous manner, it isthus possible to improve the strength properties of plate heatexchangers in the case of use of high- and ultrahigh-strength aluminumalloys.

The invention further relates to a plate heat exchanger comprising twoplates of a metallic material, wherein channel structures are formed inat least one plate and the plates are bonded to one another by solderingoutside the channel structures, characterized in that eutecticmicrostructures having a longest dimension of less than 50 micrometersare formed in the solder layer. As already elucidated above, such amicrostructure leads to a permanently stable bond of the two plates andhence to a fatigue-resistant plate heat exchanger with long lifetime.

Preferably, the eutectic microstructures are formed in solderaccumulations in the transition region from the abutted plates to thechannel structures. This region is subject to particularly high stressesin operation when a cooling medium is passed through the channelstructures. Consequently, formation of the microstructure at this pointleads to improved resistance to cracks and splitting apart in thissensitive region.

The plate heat exchanger has preferably been produced by a process ofthe invention.

The plate heat exchanger has preferably been produced by internalpressure. With regard to the associated advantages, in order to avoidrepetition, reference is made to the above elucidations. The same istrue of the features described hereinafter.

Furthermore, one of the plates is advantageously a base plate and one ofthe plates is a forming plate in which the channel structures areformed.

The base plate here preferably has a thickness of 0.5 to 5 millimeters,preferably 1 to 2 millimeters.

The forming plate preferably has a thickness of 0.2 to 2 millimeters,preferably 0.8 to 1 millimeter.

In addition, in a preferred embodiment of the plate heat exchanger, atleast one of the plates has a yield point Rp0.2 in the tensile test ofmore than 100 MPa, preferably more than 140 MPa, more preferably morethan 160 MPa.

The figures show:

FIG. 1 a working example of a process of the invention

FIG. 2 a time-temperature diagram of a process of the invention

FIG. 3 a representation of the microstructure in the solder layer of aplate heat exchanger of the invention

FIGS. 1 and 2 show a process of the invention for production of a plateheat exchanger and the associated temperature progression in schematicform. First of all, a base plate 1 and a forming plate 2 are provided.The plates here are of a 3000 series aluminum alloy, for example analloy designated 3003, 3005, 3903 or 3905. The thickness of the baseplate 1 here is 1.3 millimeters to 2 millimeters. The thickness of theforming plate 2 is 0.5 millimeter to 1.5 millimeters. The forming plate2 has been plated with a solder material. A connection opening 3 isgenerated in the base plate 1. A connecting element 4 together with asolder ring 5 is disposed at the connection opening 3.

The base plate 1 and the forming plate 2 are brought into abutment andintroduced into a preheated mold 6. The mold 6 has been preheated to atemperature of about 630° C. The mold 6 comprises an upper mold 7 and alower mold 8. Cavities 10 have been introduced into the mold surface 9of the upper mold 7, corresponding to the channel structures provided inthe plate heat exchanger to be produced. The lower mold 8 has receivingopenings 11 for the connecting element 4. Feed channels 12 for a medium14 for internal pressure forming are connected to the receiving openings11.

Then the mold 6 is closed. This corresponds to time t0 in the diagram inFIG. 2. The heat provided in the mold 6 is transferred to the plates 1,2, which are thus heated.

At a time t1, an envisaged first temperature T₁ at which the internalpressure forming is started is attained. The specific time depends onthe temperature of the mold 6 and the material and thickness of theplates 1, 2. In the case of plates 1, 2 of aluminum, owing to its goodthermal conduction properties, the time span between t0 and t1 is only afew seconds. The first temperature T₁ in this working example is 300° C.to 350° C.

For the internal pressure forming, a medium 14, preferably an inert gassuch as nitrogen, is guided under pressure between the plates 1, 2through the feed channel 12, the connecting element 4 and the connectionopening 3. This deforms the forming plate 2 until it becomes abuttedwith the mold surface 9 of the upper mold 7. The mold 6 also subjectsthe plates 1, 2 to a first pressure p₁ in order to counteract theinternal pressure of the medium 14 and in order to seal the resultantchannel structures. Adjacent to the cavities 10 of the mold surfaces 9,sealing beads 13 are provided, which ensure that the forming operationtakes place exclusively in the region of the cavities 10. Thus, highdimensional accuracy and forming precision are assured.

When the forming operation is complete (time t2), the plates 1, 2 areheated further. At about 560° C., in this illustrative processprocedure, the solder material begins to melt. At a time t3, the secondtemperature T₂ is attained. The time span required for heating dependsagain on the materials used. The second temperature T₂ here is from 600°C. to 615° C., preferably about 610° C. The solder bonding of the plates1, 2 commences with application of heat and a second pressure p₂ throughthe mold 6. This cohesively bonds the fully abutted regions of theplates 1, 2. At the same time, the connecting element 4 is cohesivelybonded to the base plate 1 by the solder ring 5.

The second pressure p₂ is lower than the first pressure p₁, in order tocounteract any possible adhesion of the aluminum plates 1, 2 to the mold6. For this purpose, the upper mold 7 and/or lower mold 8 have/has aspring mount.

At a time t4, the solder bonding is complete. Thereafter, the mutuallybonded plates 1, 2 are cooled back down to room temperature (time t6).For this purpose, the plate heat exchanger 15 is removed from the mold 6and either transferred to a cooling mold or placed in a cooling frame.

More particularly, the medium for the internal pressure forming may beused for cooling or for assistance of the cooling process. In the caseof use of nitrogen as inert medium, there is the additional advantagethat the formation of an oxide layer is prevented. Moreover, the coolingis effected somewhat more evenly and reduces deformation of the platesthat can be caused by internal stresses.

There may optionally be further heat treatment steps during this periodor thereafter.

At time t5, the solder material has solidified. The time span between t4and t5 is preferably very short and is only a few minutes or evenseconds. The effect of the short cooling time before soldersolidification is that eutectic microstructures having a longestdimension of less than 50 micrometers are formed in the solder layer.

FIG. 3 compares two sections. The left-hand image shows an accumulationof solder in the transition region between the channel structures andthe abutted plates 1, 2 that would have been in the plane of the drawingon the right of the figure. This solder bond was generated in aconventional oven soldering process with long cooling times. Eutecticmicrostructures have formed, which are elongate or acicular and have alongitudinal extent of 200 micrometers or more.

By contrast, the right-hand image shows a solder layer that has beenproduced by a process of the invention. The corresponding eutecticmicrostructures are formed in the dark regions. These have much finergrains than in the left-hand image with extents of less than 50micrometers. The effect of the fine-grain microstructure is that thesolder bond is much more stable and fatigue-resistant. This is importantespecially in the region of the transition between the channelstructures and the soldered regions, since high stresses that can leadto cracks and splitting-apart of the two plates 1, 2 occur there inoperation.

1. Process for producing a plate heat exchanger having the followingsteps: providing two plates of a metallic material, wherein the platesare brought into abutment with a solder material disposed between theplates, heating the plates to a first temperature, inserting the platesinto a mold having mold surfaces that have cavities for envisagedchannel structures, forming channel structures by local internalpressure forming of at least one plate under pressurization by the mold,heating the plates to a second temperature, solder bonding of the platesin the abutted areas.
 2. Process according to claim 1, wherein the moldis heatable and the heating to the first temperature is effected in themold.
 3. Process according to claim 1, wherein the solder bonding of theplates is likewise effected in the mold under pressurization by themold.
 4. Process according to claim 2, wherein the mold is preheated,especially to a temperature of 550° C. to 700° C., preferably to atemperature of 600° C. to 650° C., more preferably to a temperature of615° C. to 625° C.
 5. Process according to claim 1, wherein the firsttemperature is 200° C. to 550° C., preferably 300° C. to 350° C. 6.Process according to claim 1, wherein the second temperature is 550° C.to 650° C., preferably 600° C. to 615° C.
 7. Process according to claim1, wherein, adjacent to the cavities of the mold surfaces and/or at thecircumference of the mold, sealing beads are provided in order to sealthe resultant channel structures in the internal pressure forming. 8.Process according to claim 1, wherein one plate is provided as the baseplate, the other plate as the forming plate, and the channel structuresare generated in the forming plate.
 9. Process according to claim 1,wherein the plates consist of an aluminum alloy, preferably of ahigh-strength aluminum alloy.
 10. Process according to claim 1, whereinone of the two plates has been provided with a plated solder layer. 11.Process according to claim 1, wherein a connection opening is generatedin at least one of the plates.
 12. Process according to claim 11,wherein a connecting element is disposed on or in this connectionopening.
 13. Process according to claim 11, wherein the medium for theinternal pressure forming is introduced through the connection opening.14. Process according to claim 1, wherein a mold is used, the moldsurfaces of which have been provided with a coating in order to preventadhesion of the plates.
 15. Process according to claim 1, wherein aseparating agent is disposed between the plates and the mold surfaces ofthe mold in order to prevent adhesion of the plates.
 16. Processaccording to claim 1, wherein, in the internal pressure forming, a firstpressure is applied by the mold and, in the solder bonding of theplates, a second pressure is applied by the mold, the first pressurebeing higher than the second pressure.
 17. Process according to claim 1,wherein the cooling period between the solder bonding and the soldersolidification is shorter than 60 seconds, preferably shorter than 20seconds, more preferably shorter than 10 seconds.
 18. Process accordingto claim 1, wherein, after the solder bonding of the plates, these arecooled down to a temperature of less than 200° C., preferably less than60° C., within a period of less than 60 seconds, preferably less than 20seconds, more preferably less than 10 seconds, and wherein the coolingis followed within less than 72 hours by a heat treatment at atemperature of 140° C. to 250° C. over a period of 20 minutes to 24hours, preferably of 20 minutes to 8 hours, more preferably of 20minutes to 2 hours.
 19. Plate heat exchanger comprising two plates of ametallic material, wherein channel structures are formed in at least oneplate and the plates are joined to one another by soldering away fromthe channel structures, characterized in that eutectic microstructureshaving a longest dimension of less than 50 micrometers are formed in thesolder layer.
 20. Plate heat exchanger according to claim 19, whereinthe eutectic microstructures are formed in solder accumulations in thetransition region from the abutted plates to the channel structures.